I have two Honeywell Round Thermostats, one for AC one for Heat (gas) both three wire. If I take leg from each transformers and connect the two together can this be used as the common, and wired to my LUXPRO 711 in a 5-wire configuration?

Be sure both XFMRS come off the same phase at the distribution panel. XFMRS change the phase 180 degrees they are stepping up or down, from primary to secondary. You wouldn't want two different phase angles... fire hazard...

Can I check phase of each using a dual input scope, making certain both outputs wave are in-phase

Does the a "neutral" on the 120 VAC primary side of the x-former translate to the secondary 24 VAC side?

Can I test for neutral w/VOM checking each of the secondary output against ground to see which is Hot?

It really doesn't matter to check phase in your applicatio.

But for interest, if the transformers have the same voltage connect the secondaries in parallel. You'll end up with 24 V or 0V. If it's 24V, the phase is identical.

That is so simple, that's right it the two are pi radians or 180 degrees out of phase they will cancel each other out. Your pretty smart. Thanks.

You can also use the series test and it's valid for different voltages. Again voltages will add or subtract. So you would end up with 0 or 48 V.